1. Field of the Invention
The present invention relates to a process for preparing a vinylphenol polymer and a stabilized composition of vinylphenol-containing polymerization raw material.
More particularly, the present invention relates to a process for preparing a vinylphenol polymer by polymerizing a vinylphenol-containing polymerization raw material or copolymerizing this raw material with other copolymerizable vinyl compounds by using a radical initiator in the presence of methanol to ensure manufacture of a vinylphenol polymer with a higher molecular weight. The present invention also relates to a stabilized composition of vinylphenol-containing polymerization raw material to which methanol is added as an indispensable component to improve storage stability over an extended period of time. The stabilized composition is particularly suitable for conducting the process.
2. Description of the Prior Art
A vinylphenol homopolymer possesses various types of reactivity and superior heat resistance inherent to a phenol polymer having phenolic hydroxyl groups. Vinylphenol copolymers have additional characteristics which are not possessed by a vinylphenol homopolymer, such as excellent adhesion, water repellency, reactivity derived from comonomers, and are useful as functional polymer materials. Specifically, these vinylphenol polymers or copolymers are used in a wide variety of applications such as a microphotoresist material, a thermosetting resin, a metal surface treating agent, an ion exchange resin, a chelate resin, and various additives. Polymers possessing an appropriate molecular weight corresponding to each of these applications are desired. Generally, the polymers with a molecular weight (which herein means a weight average molecular weight unless otherwise specifically designated) of several thousands or more, specifically from eight thousand to one hundred thousand, are desired in the field of electronics, such as photoresist materials.
However, it is very difficult to control the molecular weight of vinylphenol polymers, because high purity vinylphenol monomer exhibits a very high reaction rate when subjected to homopolymerization of copolymerization with other vinyl compounds using a radical initiator. The reaction rate can be controlled only with difficulty. The reaction sometimes proceeds violently and recklessly. Therefore, several processes for polymerizing vinylphenol monomer while controlling the polymerization reaction have been proposed, such as a process of polymerizing a crude vinylphenol raw material containing phenolic impurities without purification (Japanese Patent Application Laid-open No. Sho 53 (1978)-13694) or a process of copolymerizing vinylphenol and an acrylic monomer or a styrenic monomer in the presence of a phenol compound possessing no unsaturated side chain and water (Japanese Patent Applications Laid-open No. Sho 61 (1986)-291606 and No. Sho 63 (1988)-130604). Although these processes can produce vinylphenol polymers with a close-to-target molecular weight with improved reproducibility of the molecular weight, the results are not always satisfactory. Rather, there are cases where the molecular weight of the polymer decreases due to a decrease in the vinylphenol concentration by dilution of the vinylphenol monomer with other components. Specifically, under the polymerization conditions where the vinylphenol monomer concentration is low, it is difficult to produce a polymer with a high molecular weight since the monomer concentration is low. In addition, because the vinylphenol monomer produces a polymer with a high molecular weight only with difficulty as compared with other common vinyl compounds, producing a high molecular weight polymer is particularly difficult even in the case of copolymerization using a high proportion of vinylphenol to a vinyl compound used as a comonomer.
Japanese Patent Application Laid-open No. Sho 51 (1976)-96882 discloses a process for controlling the molecular weight of a vinylphenol polymer by causing a compound possessing an alcoholic hydroxyl group to be present when vinylphenol is polymerized. However, this process has an object of obtaining a polymer with a molecular weight of from several hundreds to several thousands at most, using the action of the compound having the alcoholic hydroxyl group to reduce the molecular weight of the polymers. In addition, the process disclosed in the reference is concerned with thermal polymerization. According to R. H. Still et al (J. Appl. Polymer Sci., Vol. 21, 1199-1213 (1977)), the thermal polymerization of vinylphenol is a cationic polymerization caused by inter-molecular or intra-molecular transfer of protons. Copolymerizability with acrylic monomers or styrenic monomers is generally inferior according to such cationic polymerization in comparison with radical polymerization. Moreover, the randomness of the resulting copolymers is poor. Thus, the process disclosed in the reference has these basic problems.
In addition to the above, a vinylphenol monomer which is a raw material for preparing a vinylphenol polymer or copolymer is very unstable. A high purity vinylphenol monomer is easily polymerized while stored even at room temperature. The vinylphenol monomer containing such partly polymerized materials is thus inappropriate as a raw material for polymerization or copolymerization as is. Therefore, a high purity vinylphenol monomer must conventionally be stored under refrigeration at a temperature of lower than 5.degree. C., preferably lower than -20.degree. C. Thus, storing the monomer for manufacturing vinylphenol polymer in an industrial scale has been difficult. Japanese Patent Publication No. Sho 51 (1976)-29137 discloses a composition comprising alkenylphenols and at least one selected from the group of alcohol compounds and phenol compounds to stabilize the monomer mixture. The effect of stabilization obtained by the process disclosed in the reference, however, is quite insufficient. As can be seen from the examples given in the reference, the effect only continues for something over ten hours, at most. Thus, instability of the vinylphenol monomer has imposed a significant restriction in the industrial manufacture of vinylphenol polymers.